In recent years, a large variety of services have been developed for wireless networks to enrich user experience by exploiting the possibility of identifying the current geographical position of the user by using ground-based positioning. In addition to commercial services, operators may be required to provide basic emergency services and to meet the minimum coverage and position accuracy requirements provided by some regulatory bodies, e.g. the FCC (Federal Communications Commission) in the US.
One way of performing ground-based positioning is to let the UEs (User Equipment) listen for different BSs (Base Stations) and determine the distance to each BS based upon time of arrival and other parameters. Using this information, the position of a UE on a 2D map can be determined if the UE can “hear”, i.e. receive signals from, and take the time measurements of at least 3 BSs. In the standardisation collaboration 3rd Generation Partnership Project (3GPP), positioning for the Radio Access Technology (RAT) Long-Term Evolution (LTE) using the so-called OTDOA (Observed Time Difference of Arrival) method is currently being standardized. The OTDOA method is illustrated in FIG. 1, and will be described in more detail below.
However, modern cellular RATs, such as LTE, are by design not well suited for positioning, due to e.g. the effective cell isolation. In order to minimize the inter-cell interference, the network planners try to achieve an as good isolation as possible between cells. From a system capacity standpoint, it is beneficial if a UE within a serving cell receives as little interference as possible, which implies that the signal strength within the serving cell from network nodes not serving the UE should be as low as possible. FIG. 2 shows three cells 208, 210 and 212 in a system with a desired cell isolation. A mobile terminal 214, located within the cell 208 is only able to receive signals from network node 202, and not from network nodes 204 or 206.
Current implementations of ground-based positioning, e.g. those standardized in 3GPP, try to achieve acceptable positioning performance by using various interference mitigation techniques both at the system level and by means of signalling design. In order to achieve acceptable positioning performance, it may then be necessary to increase the measurement time, the measured bandwidth, or to introduce special low-interference radio resources, e.g. subframes, dedicated for positioning, during which some transmission restrictions apply, e.g. concerning user data transmissions or signalling, and during which the positioning measurements are performed.
The purpose of these alternative solutions is thus to, despite low signal levels from many BSs, make sure that strong enough signals can be received by a UE from the necessary number of BSs, so that the time-of-arrival of the received signals can be accurately estimated, or similarly, for UL (UpLink) positioning, make sure that a sufficient number of BSs receive strong enough signals from a UE, so that the BSs can estimate the time-of-arrival of the received signals with high enough precision. This area, however, still leaves room for further improvements.